Cleaning method

ABSTRACT

A cleaning method which reduces the quantity of use of a chemical solution and ultrapure water by simplifying the cleaning process, has an extremely high cleaning effect, and does not damage a semiconductor substrate. This cleaning method comprises a first step of removing organic matters, metals, and fine particles adhering to a substrate body by ultrapure water containing ozone, a second step of applying megasonic to a solution of a mixture of ultrapure water and hydrofluoric acid in a cleaning bath to remove the metals and the fine particles adhering to the substrate body, a third step of removing the chemical solution used in the second step, and a forth step of applying megasonic to the ultrapure water in the cleaning bath to clean the substrate body.

TECHNOLOGICAL FIELD

The present invention relates to a cleaning method, and morespecifically to a method of cleaning a semiconductor substrate.

BACKGROUND TECHNOLOGY

A process of cleaning a semiconductor substrate is one of the mostimportant processes in a semiconductor device production process, andits importance has been increasingly higher as performance and a degreeof integration in semiconductor device have also been becoming higher.

Conventionally, sulfuric acid, chloric acid, ammonia, hydrogen peroxide,hydrofruolic acid, or a mixture thereof have been used for removingimpurities adhering to a surface of a semiconductor substrate. But ithas been found as a result of a research conducted by the presentinventors that the impurities can not completely be removed with thesechemicals, and also that a surface of a semiconductor is corroded bythese chemicals and minute irregularities are generated on the surface,and it has been confirmed that these phenomena give effects to devicecharacteristics such as mobility of carrier and insulating performanceof an oxide film formed on the surface. Accordingly, it has been desiredto develop a cleaning method enabling complete removal of impuritieswithout giving any damage to the semiconductor substrate for productionof semiconductor devices with higher performance.

In addition, as the cleaning method based on the conventional technologyrequired a long time and the cleaning steps are very complicated, andalso as a vast quantity of chemicals or ultra-pure water is used, thecost is very large, and large scale facilities are required, so thatprice reduction of semiconductor devices has been prevented.

To solve the problems as described above, it is an object of the presentinvention to provide a cleaning method with extremely excellent cleaningeffect which enables reduction in a quantity of chemicals and ultra-purewater used for cleaning and also which give no damage to a semiconductorsubstrate.

DISCLOSURE OF THE INVENTION

The cleaning method according to the present invention comprises a firststep of removing organic materials, metal, particles adhering to asubstrate with ultra-pure water containing ozone; a second step ofremoving metal or particles adhering to the substrate by irradiatingmegasonic to a chemical comprising ultra-pure water containinghydrofluoric acid in a cleaning bath; a third step of removing thechemical used in the second step, and a fourth step of cleaning thesubstrate by irradiating megasonic to the ultra-pure water in thecleaning bath.

Gasses result therein should preferably be removed from the ultra-purewater used in the second step and/or the fourth step.

All the first to fourth steps are performed under a room temperature.Further, the third step is characterized in that the chemical is removedby showering or spraying ultra-pure water.

The present invention is advantageously applied to a case where theultra-pure water used in the second step contains hydrofluoric acid,hydrogen peroxide, or a surfactant.

It is preferable that the second and/or the fourth step are based on asystem in which ultra-pure water in a cleaning bath withdrawn keeping aflow of the ultra-pure water therein at a constant speed and thatmegasonic is irradiated from a top of the cleaning bath. Especially, afrequency of the megasonic should preferably be in a range from 0.8 to10 MHz.

It is desirable that the first to fourth steps described above arecarried out in an inactive atmosphere with the purity of 99.9999 ormore, and also it is desirable that concentration of ozone in theultra-pure water containing ozone therein is in a range from 2 to 10ppm.

Function

With the present invention, impurities adhering to a semiconductorsubstrate such as organic materials, metal, or particles can be removed.Further when cleaning is performed according to the present invention,minute irregularities are not generated on the surface at all. Thereason is presumably as described below.

At first, by using ultra-pure water with ozone added therein in thefirst step, organic materials adhering to a surface of a semiconductorare completely oxidized and decomposed and at the same time thesemiconductor surface is mildly oxidized, so that also a portion ofmetal or particles is fetched into the oxide film. In the second step,the oxide film is removed by hydrofluoric acid, and at the same timeimpurities such as metal fetched into the oxide film are removed. Theremoval of impurities is enhanced when megasonic irradiation is appliedto the hydrofluoric acid, whereby the hydrofluoric acid vibrates whilecontacting the semiconductor surface, so that removal of particles isexecuted speedily. It is also conceivable that hydrogen bond in water isdisconnected causing changes in the physical properties such asviscosity and removal of impurities is promoted further. In this step,it is possible to remove an oxide film with hydrofluoric acid by adding,in addition to hydrofluoric acid, hydrogen peroxide and a surfactant tooxide the surface with the hydrogen peroxide, and for this reason, evenif a quantity of impurities, removal of impurities can easily beexecuted.

In the third step, hydrofluoric acid or surfactant used in the secondstep and remaining on a substrate is rinsed with ultra-pure water toclean it away. This rinsing should preferably be executed with a showeror a spray so that ultra-pure water always contact a surface of asemiconductor. In the fourth step, which is a final step, megasonic isirradiated onto ultra-pure water to completely clean off a minutequantity of the chemical or chemicals not removed away by means ofshowering or spraying. As described above, it is conceivable that, dueto vibration of ultra-pure water, physical properties of water arefurther changed, and mutual reaction between remaining components of achemical and water is further promoted, and the semiconductor surface iscompletely cleaned.

It should be noted that, if megasonic irradiation is executed in thesecond step or in the third step and a dissolved gasses is presenttherein, the dissolved gasses may be changed to ordinary gasses causingnon-uniformity in cleaning, and for this reason it is desirable that thedeaeration should be executed previously to remove the dissolved gasses.A deaerating apparatus with UF (ultrafiltration film) may be used fordeaeration for removing dissolved gasses. In this case, even if a minutequantity of particles or other foreign materials is present in theultra-pure water, the foreign materials can be removed simultaneouslywith the UF film.

Further, it is preferable to match a direction in which the ultra-sonicproceeds to a direction in which the ultra-pure water flows, and forthis reason, it is desirable to irradiate megasonic from the upper sideby keeping a uniform flow of ultra-pure water and withdrawing theultra-pure water downward (from the upper side to the lower side). Withthis operation, the cleaning effect is further improved. Further, afrequency of the megasonic should preferably be in a range from 0.8 MHzto 10 MHz. By setting the frequency to this range, the cleaning effectis further improved.

It is preferable to execute the cleaning steps according to the presentinvention in an inactive gas atmosphere free of impurities. This is forevading intrusion of impurities from the atmosphere and also forpreventing generation of a oxide film after the oxide film is removed inthe second step. As an inactive gas, for instance, a N₂ gas or an Ar gasmay be used, and a concentration of impurities in the inactive gasshould preferably be 1 ppb or less.

A concentration of ozone in ultra-pure water with ozone added therein,which is used in the first step, should preferably be in a range from 2to 10 ppm. If the concentration is 2 ppm or less, oxidization of organicmaterials may be inadequate, while, if the concentration is 10 ppm ormore, a thickness of the oxide film is too large, which in turn requiresa long period of time for removal thereof and results in increase of asurface roughness. Further, the ultra-pure water may be withdrawndownward like in the fourth step to realize a uniform flow of theultra-pure water. Further, the megasonic may be irradiated from theupper side. In the first step, different from other steps, dissolvedozone is precipitated even if megasonic is irradiated, but as an oxidefilm is generated on the surface, and bubbles never reside on thesurface.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a concept drawing showing a cleaning line according toEmbodiment 1 of the present invention.

(Description of Signs)

101: Feed water tank

102: Pre-processing device

103: Primary pure water tank

104: Reverse osmosis module and ion-exchange column

105: Ultra-pure water tank

106: UF deaerating device

107: Cleaning bath for first step

108: Ozone generator

109: Cleaning bath for second step

110: Cleaning bath for third step

111: Cleaning bath for fourth step

112, 112': Oscillator for generation of megasonic1

13: Shower

114, 114', 114", : Pump

115: Recovery line

116: Tank for hydrofluoric acid processing

BEST MODE FOR CARRYING OUT THE INVENTION

Next description is made for the present invention with reference toembodiments thereof.

EMBODIMENT 1 AND COMPARATIVE EXAMPLE 1

(Embodiment 1)

A cleaning line in which cleaning according to the present invention isapplied is shown in FIG. 1.

In FIG. 1, designated at the reference numeral 1 is a feed water tank,at 102 a pre-processing device, at 103 a primary water tank, at 104 areverse osmosis module and an ion-exchange column, at 105 a ultra-purewater tank, at 106 a UF deaerating device, at 107 a cleaning bath forthe first step, at 108 an ozone generator, at 109 a cleaning bath forthe second step, and at 110 a cleaning bath for the third step, which isa shower cleaning bath. The cleaning liquid is sent to a tank forprocessing 116 with hydrofluoric acid, and hydrofluoric acid is reactedtoo calcium carbonate and is recovered as calcium fluoride. Thereference numeral indicates a cleaning bath for the fourth step, and thecleaning liquid is circulated via a pump 114 and a recovery line 115 toa primary ultra-pure water or to the feed raw tank 101. In a case ofthis cleaning line, around 80 percent of ultra-pure water used can berecovered under the following conditions. The reference numerals 112,112' indicate an oscillator for generating megasonic.

Using the cleaning line shown in FIG. 1, 25 sheets of 5-inch wafer areaccommodated in a wafer carrier made of Teflon, and are successivelycarried to the cleaning baths in the first to fourth steps, when thewafers are subjected to processing for cleaning with respective cleaningliquids.

It should be noted that a quantity of a chemical in the cleaning bathfor the third step is 7 l. Further, the cleaning conditions are asflows:

First step: The sample is dipped in 5 l ultra-pure water with ozoneadded therein (ozone concentration of 10 ppm) for 3 minutes and thencleaned.

Second step: The sample is dipped and cleaned in a mixture ofhydrofluoric acid, hydrogen peroxide, and ultra-pure water with also asurfactant (200 ppm) added therein (0.03:1:2) for 10 minutes irradiatinga ultra-sonic(1 MHz, 1200 W).

Third step: Cleaning is executed for 10 minutes with a ultra-pure watershower having a low rate of 1.5 l/min.

Fourth step: Cleaning is executed for 10 minutes irradiating ultra-sonic(1 MHz, 1200 W) to ultra-pure water flowing at a flow rate of 1.5 l/min.

The ultra-pure water in the fourth step is ultra-pure water withdissolved gasses having been removed therefrom, using a UF deaeratingdevice. Further, the oscillator for generation of megasonic was placedon a top of the cleaning bath, and the ultra-pure water was withdrawnfrom the bottom of the cleaning bath. Further, in the fourth step,ultra-pure water was withdrawn from the bottom of the cleaning bath.

After each sample was cleaned according to the cleaning method accordingto the present invention, quantities of organic materials, metal, andparticles were measured with FTIR (made by Vio-Rad), TRXRF (made byTechnos), and WIS150 (made by Canon) respectively. The result is shownin Table 1.

Further, a quantity of ultra-pure water used for cleaning and a quantityof chemicals are shown in Table 2.

COMPARATIVE EXAMPLE 1

Quantities of organic materials, metal, and particles adhering to awafer surface after cleaned according to the conventional cleaningmethod were measured for comparison like in Embodiment 1. A result ofmeasurement and a quantity of ultra-pure water used for cleaning and aquantity of each chemical are shown in Table 1 and Table 2 respectively.

The cleaning conditions in the comparative example are as shown below.

    ______________________________________    1:    Sulfuric acid + hydrogen peroxide (4:1)                                 5 minutes    2:    Rinsing with ultra-pure water (1.5 1/min)                                 5 minutes    3:    Sulfuric acid + hydrogen peroxide (4:1)                                 5 minutes    4:    Rinsing with ultra-pure water (1.5 1/min)                                 5 minutes    5:    Sulfuric acid + hydrogen peroxide (4:1)                                 5 minutes    6:    Rinsing with ultra-pure water (1.5 1/min)                                 10 minutes    7:    Hydrofluoric acid + ultra-pure water +                                 1 minute          hydrogen peroxide (0.03:1:2)    8:    Rinsing with ultra-pure water (1.5 1/min)                                 10 minutes    9:    Sulfuric acid + hydrogen peroxide (4:1)                                 5 minutes    10:   Rinsing with ultra-pure water (1.5 1/min)                                 10 minutes    11:   Hydrofluoric acid + ultra-pure water +                                 1 minute          hydrogen peroxide (0.03:1:2)    12:   Rinsing with ultra-pure water (1.5 1/minute)                                 10 minutes    13:   Ammonia + hydrogen peroxide + ultra-pure                                 10 minutes          water (0.05:1:5)    14:   Rinsing with ultra-pure water (1.5 1/min)                                 10 minutes    15:   Hot ultra-pure water (1.5 1/min)                                 10 minutes    16:   Rinsing with ultra-pure water (1.5 1/minute)                                 10 minutes    17:   Hydrofluoric acid + ultra-pure water +                                 1 minute          hydrogen peroxide (0.03; 1:2)    18:   Rinsing with ultra-pure water (1.5 1/min)                                 10 minutes    19:   Chloric acid + hydrogen peroxide +                                 10 minutes          ultra-pure water (1:1:6)    20:   Hot ultra-pure water (1.5 1/min)                                 10 minutes    21:   Rinsing with ultra-pure water (1.5 1/min)                                 10 minutes    22:   Hydrofluoric acid + ultra-pure water +                                 1 minute          hydrogen peroxide (0.03:1:2)    23:   Rinsing with ultra-pure water (1.5 1/min)                                 10 minutes    ______________________________________

                  TABLE 1    ______________________________________             Quantity of Residual Impurities after Cleaning             Quantity of                       Quantity of                                  Quantity of             organic materials                       metal (Cu) particles             (CH.sub.2) (mol/cm.sup.2)                       (atom/cm.sup.2)                                  (>0.1 μm)    ______________________________________    Comparative               3.3 × 10.sup.14                           5 × 10.sup.10                                      9    example 1    Embodiment 1               0*          5 × 10.sup.8                                      0 ˜ 1    ______________________________________     *Below limit for detection

                  TABLE 2    ______________________________________                   Ultra-pure water                             Chemical    ______________________________________    Comparative example 1                     180 L       70 L    Embodiment 1     34.5 L       7 L    ______________________________________

As clearly understood from Table 1, it has been found that the cleaningmethod according to this embodiment is effective in cleaning any typesof impurities including organic materials, metal, and particles.Further, as shown in Table 2, a quantity of the ultra-pure water and aquantity of chemicals use in the present embodiment are 1/5 or less ascompared to those in the conventional technology, which in turn resultin substantial reduction of cleaning cost, and also that a period oftime required for cleaning is also reduced.

Embodiment 2

Cleaning was executed under the same conditions as those in Embodiment 1excluding the fact that frequencies of the megasonic 112, 112' werechanged, and impurities on a wafer surface were measured. A result ofthe measurement is shown in Table 3.

                  TABLE 3    ______________________________________           Quantity of Residual Impurities after Cleaning             Quantity of   Quantity of                                     Quantity of             organic materials                           metal (Cu)                                     particles    frequency             (CH.sub.2) (mol/cm.sup.2)                           (atom/cm.sup.2)                                     (>0.1 μm)    ______________________________________    0.7 MHZ  1.5 × 10.sup.14                             6 × 10.sup.9                                     4 ˜ 5    0.8 MHz  0             1.7 × 10.sup.8                                     1 ˜ 2      1 MHz  0               1 × 10.sup.8                                     0 ˜ 1     10 MHz  0             1.2 × 10.sup.8                                     0 ˜ 1     12 MHZ    3 × 10.sup.13                           1.5 × 10.sup.8                                     1 ˜ 2    ______________________________________

As clearly understood from Table 3, it has been found that frequenciesof the megasonic in a range from 0.8 to 10 MHz shows the excellentcleaning effect.

Embodiment 3

As a cleaning liquid used in the second step in Embodiment 1,hydrofluoric acid diluted with ultra-pure water to 5% and with dissolvedgasses removed with a UF deaerating device was used, 1 MHz megasonic wasirradiated from a top of the cleaning bath, and cleaning was performedwithdrawing the ultra-pure water from the bottom of the cleaning bath ata flow rate of 1.5 l/min. Operations were executed like in Embodiment 1excluding the second step.

Impurities on the wafer surface after cleaning were measured like inEmbodiment 1, and any of organic materials, metal, and particles was notdetected, and it was found that the cleaning method according to thepresent invention further improves the cleaning effect.

Industrial Applicabilities

The present invention can provide a cleaning method which enablecomplete removal of impurities adhering to a surface of a semiconductorsuch as organic materials, metal, an particles without giving damages tothe semiconductor substrate. Further, the processing sequence is quitesimple, so that the processing can be completed within a short period oftime, and further the stable cleaning effect can be obtained.Furthermore, quantities of ultra-pure water and chemicals are largelyreduced, so that low-price semiconductors can be realized.

In addition, a number of required cleaning baths can be reduced ascompared to the conventional technology, so that an area of the wet lineitself can be reduced to around 1.3 of that in the conventionaltechnology.

I claim:
 1. A cleaning method comprising:a first step of removingorganic materials, metal and particles adhering to a substrate body withultra-pure water containing ozone in a cleaning bath; a second step ofremoving metal and particles adhering to the substrate by irradiatingmegasonic to a chemical comprising ultra-pure water containinghydrofluoric acid in a cleaning bath; a third step of removing thechemical used in said second step; and a fourth step of cleaning thesubstrate by irradiating megasonic to ultra-pure water in a cleaningbath.
 2. The cleaning method according to claim 1 wherein the ultra-purewater used in at least one of the second step and the fourth step isthat from which dissolved gases have been removed.
 3. The cleaningmethod according to claim 1 wherein any of the steps is executed at roomtemperature.
 4. The cleaning method according to claim 1 wherein thechemical in said third step is removed from the substrate body by aspray of ultra-pure water.
 5. The cleaning method according to claim 1wherein the ultra-pure water used in said second step contains hydrogenperoxide and a surfactant.
 6. The cleaning method according to claim 1wherein the ultra-pure water in one of said second step and fourth stepis flowed at a constant speed and withdrawn downward over the substratebody while megasonic is irradiated from a top portion of the cleaningbath.
 7. The cleaning method according to claim 1 wherein a frequency ofthe megasonic is in the range of approximately 0.8 to 10 Megahertz. 8.The cleaning method according to claim 1 wherein any of the steps arecarried out in an inactive atmosphere with the purity of 99.9999% ormore.
 9. The cleaning method according to claim 1 wherein theconcentration of ozone in said ultra-pure water containing ozone thereinin a range of approximately 2 to 10 ppm.
 10. The cleaning methodaccording to claim 1 wherein the flow of ultra-pure water containingozone in the cleaning bath of the first step is kept at constant flowspeed, withdrawn downward over the substrate and irradiated withmegasonic from a top portion of the cleaning bath.